This invention generally relates to modulated carrier communications networks and more specifically to networks which require a data transmitter and receiver to operate synchronously.
Modulated carrier data communications networks convey data between a central site and one or more remote sites over telephone lines or other communications links. The central site typically comprises a digital data transmitter and receiver, such as a digital computer and its associated input/output control system. Each remote site also comprises a digital data transmitter and receiver, such as a peripheral unit.
At the central site, a modulator circuit receives the digital data from the central site transmitter and converts the data to appropriately modulated carrier signals for transmission over a data transmission path to a designated remote site. At the remote site, a demodulator circuit receives the incoming modulated carrier signal and converts it back to its original digital form for use by the data receiver. Similarly, a data transmitter at a remote site supplies data to its modulator for conversion to modulated carrier signals for transmission back to the central site where a demodulator converts the incoming modualted carrier signal back into digital form for the data receivers at the central site. Generally, a modulator circuit and a demodulator circuit at each site are packaged together in a "modem."
A single data transmitter and receiver at a central site may communicate with one or more remote sites, or "drops" over one or more separate communications links. Each communications link couples a central modem and a number of drops. Appropriate circuits at the central site and each remote site enable any one remote site to communicate with the central site at any given time.
In these networks, the digital data passes between digital equipment and modems at both the central and remote sites in serial fashion as a stream of digital bits. Some networks also include time division multiplexers which shift the digital data connections from a single modem and to one of a pair of digital data transmitters and receivers at a site as successive bits are received or transmitted. It is imperative, therefore, that the circuits which process the digital bits stay in synchronism with each other. If the transmitting and receiving modems lose synchronism, the resulting decoded signals at the receiving modem will be interpreted improperly.
This "digital synchronism" is above and beyond the synchronism which is required between oscillators and other circuits which actually modulate and demodulate the carrier signal. All these circuits in a polled network, for example, could be operating properly and the network could still lack digital synchronism. These are several conditions which may cause a lack of digital synchronism. Obviously, when a data communications network is first energized, there is no digital synchronism. Electrical disturbances or other problems in a power supply may cause a power interruption or loss or addition of a clock pulse thereby causing the receiving modem to "lose" a digital data bit or insert a false bit. In a time-division multiplexed network, this can result in a reversal of connections to the digital data receivers so the digital data is routed improperly.
Data communications networks usually include circuits for synchronizing initially the modulators and demodulators and then maintaining the demodulators in synchronism with the modulators. These circuits operate, in part, in response to digital signals which are assumed to be random signals. However, a message conveyed by the system might, from time to time, produce a repetitive pattern and the synchronizing circuits at the receiver may respond to this repetitive pattern by losing synchronism. Therefore, prior communications networks incorporate randomizing circuits, such as pseudo-random number generators, for scrambling and unscrambling the incoming digital data to minimize generation of repetitive patterns. If digital synchronism between the pseudo-random number generators at the transmitting and receiving modems is lost, improper decoding occurs at the receiving modem and data errors result.
Prior synchronizing methods prohibit the transfer of any data during initial synchronizing operations. Typically, these synchronizing operations occur only during a "training" period, typically usually when an entire network is energized initially. Periodic network-wide synchronizing operations also might be conducted or unique data codes sent in other arrangements. In one approach, the incoming data at a transmitting modem is maintained at one value (e.g., zero) and incoming signals at a receiving modem are routed to a shift register in the pseudo-random number generator which is disabled during the training period. After some predetermined number of bit intervals, registers in both pseudo-random number generators should have the same state. If they do, there is synchronism and normal operations can begin.
If a remote site does not acquire digital synchronism or if it is turned off and, some time later is turned on, that site, in a polled network, is deleted from a polling list. When the omission from the polling list is observed, personnel at the central site may then initiate another network-wide synchronizing operation. In all these approaches, however, the digital synchronizing operation is initiated manually and excludes any data transfers while it takes place. Thus, the overall effective data transfer rate is reduced below the maximum network capability. The magnitude of the reduction will depend on the time required for the synchronizing operation.
These networks also are subject to a false recognition of an acquisition signal which designates the initial training interval during which the synchronizing and other control operations occur. This can cause a receiving site to effect a synchronizing operation improperly.
Therefore, it is an object of this invention to provide a data communications network wherein synchronizing operations are performed simultaneously with normal data transmissions.
Another object of this invention is to provide a data communications network wherein digital synchronizing operations do not reduce the effective data transfer rate of the system.
Yet another object of this invention is to provide a data communications network in which synchronizing operations occur automatically.
Still another object of this invention is to provide a data communications network in which synchronizing operations utilize randomizing circuits for controlling the synchronizing operations.